This invention relates to impedance-matching devices in general, and in particular to devices for indicating the proper adjustment of a frequency-compensated attenuation probe.
Attenuation probes are generally associated with oscilloscopes; however, they may be used as a means for coupling an input signal to any electronic test and measurement instrument. A typical method of compensating the probe is to connect it to an oscilloscope and apply a precise square-wave reference signal thereto while viewing the resultanting signal on the oscilloscope screen. Distortions in the square-wave signal produced by the impedance mismatch are indicated on the wave form by either peaked or a rolled-off leading edge of the square-wave. The probe may be adjusted while watching the display to adjust the square-wave to a square leading corner.
Other electronic instruments having wide-band capability, such as events counters and the like, have come into wide usage, and such instruments lend themselves to usage of an attenuation probe therewith to reduce the amplitude of an input signal to a usable level. However, these instruments do not have a display device for viewing an input waveform and it is difficult to properly compensate the probe without the use of an oscilloscope. One solution to this problem has been described in U.S. Pat. No. 4,070,615, which patent is assigned to the assignee of the present invention. This patent teaches the application of a square-wave signal to both inputs of a differential amplifier, one input receiving the signal directly and the other input receiving the signal via an attenuation probe and input circuitry of an electronic instrument. Any impedance mismatch causes an output from the differential amplifier which is rectified and filtered to produce a control voltage to be applied to an indicator light. A drawback to this system is that the indicating circuitry has the undesirable effect of loading down the input. Furthermore, proper compensation of the probe is indicated when the light is its brightest, and thus accuracy is dependent upon the judgment of the person compensating the probe.